Multiple-input, multiple-output (MIMO) communications are well-known techniques for improving the capacity and reliability of a wireless communication channel. A conventional MIMO wireless system uses multiple transmit antennas and multiple receive antennas to provide a linear increase in capacity with K, where K is the minimum of number of transmit (M) antennas and receive antennas (N) (i.e., K=min(M,N)). By way of example, a conventional 4×4 MIMO system transmits four different data streams separately from four transmit antennas of a base station. The four transmitted signals are received at the four receive antennas of a subscriber station.
The subscriber station (SS) then performs some form of spatial signal processing on the received signals in order to recover the four data streams. By way of example, the subscriber station (or user device) may perform a spatial signal processing technique known as V-BLAST, which uses successive interference cancellation principle to recover the transmitted data streams. Other variants of MIMO techniques may include some type of space-time coding across the transmit antennas (e.g., D-BLAST) or may include a beamforming technique, such as spatial division multiple access (SDMA).
In the case of a single-code word MIMO transmission, the base station (BS) adds a cyclic redundancy check (CRC) block to a single data block and then performs coding and modulation on the combined CRC and data blocks. The coded and modulated symbols are then demultiplexed for transmission over multiple antennas. In the case of multiple-code word MIMO transmission, the base station demultiplexes a data block into smaller data blocks and attaches individual CRC blocks to the smaller data blocks. The base station then performs separate coding and modulation operations on the smaller combined CRC and data. The smaller data and CRC blocks are then transmitted via separate MIMO antennas or beams.
It should be noted that in case of multi-code word MIMO transmissions, different modulation and coding techniques may be used on each of the individual streams, resulting in a so-called PARC (per antenna rate control) scheme. Also, multi-code word transmission allows for more efficient post-decoding interference cancellation, because a CRC check can be performed on each of the code words before the code word is cancelled from the overall signal. In this way, only correctly received code words are cancelled, thereby avoiding any interference propagation in the cancellation process.
Hybrid acknowledge request (ARQ) is a retransmission technique whereby the transmitter sends redundant coded information (e.g., parity bits in turbo coding) in small increments (or subpackets). The subpackets are generated at the transmitter by first performing channel coding on the information packet and then breaking the resulting coded bit stream into smaller units called subpackets. For example, an original data packet P and the corresponding parity bits may be broken into subpackets SP1, SP2, SP3, . . . , SPn. The receiver tries to decode the information and recover the original data packet P after receiving the first subpacket SP1. In case of unsuccessful decoding, the receiver stores the SP1 and sends a NACK message to the transmitter.
After receiving the NACK message, the transmitter transmits the second subpacket SP2. After receiving the second subpacket, the receiver combines subpacket SP2 with the previously stored subpacket SP1 and jointly decodes subpackets SP1 and SP2 in order to recover original data packet P. At any point, if the information packet is successfully decoded (e.g., by a successful cyclic redundancy check (CRC) operation), the receiver sends an ACK message to the transmitter. After receiving an ACK message, the transmitter moves on to the transmission of a new information packet to the same or a different subscriber station (or user).
One of the disadvantages of a single-user MIMO PARC transmission scheme is that multiple channel quality indicator (CQI) feedback estimates are required for each of the individual streams. This requires excessive signaling overhead and results in system inefficiency. In a multi-user MIMO system, it is possible to implement a PARC transmission scheme with just one CQI feedback estimate per subscriber station. In such a case, each subscriber station (or user) reports the best CQI estimate determined by using, for example, an MMSE algorithm along with the MIMO stream identity.
A multi-user MIMO system requires that a large number of subscriber stations are present in the system, so that each subscriber station can be selected for transmission when it experiences the best channel quality. If the number of subscriber stations in the system is small, the system is less likely to find subscriber stations experiencing peak channel conditions. This degrades the performance of a multi-user MIMO scheme. In the presence of a small number of subscriber station, it is advantageous to schedule multiple MIMO streams to the same subscriber station using single-user MIMO transmission mode. The number of subscriber stations with traffic buffers that are not empty varies dynamically due to packet data traffic burst characteristics. It should be noted that, for single-user MIMO mode, multiple CQI feedback values are required, while in multi-user MIMO mode, a single CQI feedback value per subscriber station may suffice.
In sum, in conventional wireless networks, a MIMO system operates either in single user (or single subscriber station) MIMO mode or in multi-user (multi-subscriber station) MIMO mode. System performance is better for the single-user MIMO case when the number of subscriber stations in the system is small. On the other hand, a multi-user MIMO system gives better performance in the presence of large number of subscriber stations.
Furthermore, the number of subscriber stations having data to receive or to transmit in a system may vary dynamically due to the bursty nature of the traffic. Subscriber stations are not aware of the dynamic traffic situation in the base station and, therefore, cannot switch the mode of CQI feedback between single-user and multi-user CQI feedback. As a result, conventional MIMO schemes result in inefficient use of system capacity and resources.
Therefore, there is a need in the art for an improved wireless network capable of dynamically switching between single-user and multi-user MIMO modes. In particular, there is a need for a wireless network capable of dynamically switching between single-user and multi-user MIMO modes with minimal signaling overhead.